c++: duplicate block-scope extern [PR 97877]

c++: duplicate block-scope extern [PR 97877]

We ICED with a duplicated block-scope extern, as duplicate_decls was
dropping the decl_lang_specific of olddecl.  Simplys adding
appropriate retrofitting and copying turned out to be insufficient
because you can get a block-scope using decl also matching the extern.
The latter seems a little suspicious and I have asked CWG for advice.
While there robustified the assert about releasing olddecls'
lang-specific -- if it had one, the new decl better have one.

	PR c++/97877
	gcc/cp/
	* decl.c (duplicate_decls): Deal with duplicated DECL_LOCAL_DECL_P
	decls.  Extend decl_lang_specific checking assert.
	gcc/testsuite/
	* g++.dg/lookup/pr97877.C: New.